1. Field of the Invention
This invention relates to instruments for measuring internal dimensions, and particularly to an instrument for measuring internal dimensions wherein an interval between two measuring elements is detected by a displacement detecting device to thereby measure an internal dimension of a workpiece to be measured.
2. Description of the Prior Art
A conventional instrument for measuring internal dimensions, e.g., an instrument for measuring internal dimensions described in Japanese Patent Application No. 54421/79, is of such an arrangement that a detecting head having a pair of probes is provided. These probes are abutted against a reference ring gauge having predetermined dimensions which rests on and is affixed to a measuring table, and an inner wall at one side of a workpiece to be measured, respectively. At the same time, subsequently, the measuring table is moved to abut the probes against the ring gauge and an inner wall at the other side of the workpiece to be measured, and an internal dimension of the workpiece is obtained based on a difference in inclination between the probes through a relative measurement therebetween as determined from the ring gauge.
As described above, according to method of measurement, a relative measurement is effected, i.e., the internal dimension of the workpiece is measured as a difference from the inner dimensions of the ring gauge having predetermined dimensions which rests on one and the same measuring table. For structural reasons relating to the instrument for measuring an internal dimension, a predetermined positional relationship is required between the workpiece and the ring gauge in their mounting, thus presenting such disadvantages as that the measuring operation is complicated and takes a long period of time because considerable labor is needed for constructing the mounting of the workpiece and the ring gauge.
To obviate the above-described disadvantages, there has been proposed the instrument for measuring internal dimensions of Japanese Utility Model Application No. 146754/79, according to which a method is further proposed as being capable of effecting an absolute measurement of an internal dimension of a workpiece without using a ring gauge. According to this method two probes are provided on a detecting head, and parallel springs are interposed between members for holding these probes and transferring means, whereby a displacement value between the respective holding members holding the two probes is detected.
Although the above-described instrument for measuring internal dimensions is simplified as compared with one in which the workpiece to be measured must be disposed at a predetermined interval with respect to a ring gauge, it requires various parts to achieve certain functions, including the function of moving probes longitudinally and laterally to obtain the true diameter when an internal dimension of the workpiece is measured the function of storing measured values at respective points, and a maximum value detecting function for indicating the maximum value as the diameter. Therefore, the need has been voiced for improvements in eliminating such troublesomeness in the measuring operation and in reducing the number of associated essential functions of the instrument for measuring internal dimensions, thus lowering the cost of production.